Cloning and functional characterization of two murine uridine nucleotide receptors reveal a potential target for correcting ion transport deficiency in cystic fibrosis gallbladder

Extracellular nucleotides regulate transepithelial ion secretion via multip le receptors. The P2Y(2) receptor is the predominant transducer of chloride transport responses to nucleotides in the airways, but the P2 receptors th at control ion transport in gastrointestinal epithelia have not been identi fied. UTP and UDP promote chloride secretion in mouse jejuna and gallbladde r epithelia, respectively, and these responses were unaffected by P2Y(2) re ceptor gene disruption. Pharmacological data suggested the involvement of P 2Y(4) and P2Y(6) receptors in gastrointestinal responses. To identify the P 2Y receptors responsible for the gastrointestinal actions of UTP and UDP, w e have cloned the murine P2Y(4) and P2Y(6) receptors and have stably expres sed each in a null cell line to examine the nucleotide-promoted inositol ph osphate formation and intracellular Ca2+ mobilization. The (m)P2Y(4) recept or was potently, but not selectively, activated by UTP (UTP greater than or equal to ATP > ITP > GTP > CTP), and it was not activated by UDP or ADP. T he (m) P2Y(6) receptor was highly selective for UDP (UDP much greater than ADP = GDP). The nucleotide selectivities observed with the recombinant (m)P 2Y(4) and (m)P2Y(6) receptors resemble those for nucleotide-promoted chlori de transport in murine P2Y(2)(-/-) jejuna and gallbladder epithelial cells, respectively. Ion transport responses to nucleotide additions were examine d in freshly excised tissues from cystic fibrosis transmembrane regulator-d eficient mice. Although the effect of UTP on jejunal short-circuit current (I-sc) was impaired in the CF mouse, UDP-promoted I-sc changes were not aff ected in CF gallbladder epithelium, suggesting that the P2Y(6) receptor is a target for treatment of CF gallbladder disease.